When attempting to determine mechanical properties of homogeneous and composite materials while testing in compression samples taken from components whose slenderness ratio is large, such as thin sheets, columns, or thin-walled tubes, difficulties arise became the Euler column buckling load (see OTHER PUBLICATIONS) will be less than that required to fully test the specimen. Whether the test piece be a metallic, a ceramic, a polymer or a composite material this problem is ever present. Also in the field of mechanics of materials research, thin hollow cylinders are utilized as test specimens to determine the validity of failure hypotheses. Such hypotheses often require multi-axial loading which includes compression stressing. Again, the researcher faces the problem that the thin hollow cylindrical specimen may fail due to Euler buckling, depending upon its slenderness ratio, prior to attaining the desired data.
To demonstrate how Euler column buckling can interfere with the determination of mechanical properties consider the following realistic example: Assume for the sake of demonstration that the exact compression strength or yield stress (or proportional limit) of a thin-walled steel component is desired after fabrication, and it is known that prior to the manufacturing process the compression yield strength is approximately 100 ksi. A test coupon is removed from the structure to determine its compression properties; it has an over all length L of 3 inches and rectangular cross-sectional dimensions of b=1/2 and t=1/16 inches. Since Py=Sy (A)=Sy (bt)=100 ksi (1/2 in.times.1/16 in)=3125 lbs. Py is the expected load when testing in compression to determine the yield strength Sy of the material and A is the cross-sectional area of the test specimen.
The critical Euler buckling load Pcr for a column, or in this case a thin rectangular specimen with clamped end conditions having an axial length of L; and cross-sectional dimensions of length b and thickness t is given by the following general equation: EQU Pcr=4.pi..sup.2 EI/L.sup.2 ( 1)
Where E, the modulus of elasticity of the steel material, which is 30 million psi and I is the moment of inertia of the cross-section. In this case: EQU I=bt.sup.3 /12 (1a)
Substitution of Eq. 1a into Eq. 1 gives: EQU Pcr=E.pi..sup.2 bt.sup.3 /3L.sup.2 ( 2)
Substitution of the above numerical values into Eq. 1 results in a critical buckling load Pcr of 1339 pounds. It is see from this example that buckling of the specimen will occur prior to obtaining the yield stress Sy of the material and thus the determination of the desired mechanical properties will not be realized.
As seen from the above discussion, when manufacturing various structural components it is desirable to test such structures for mechanical strength. For example, in the airplane industry, some of the airplane parts are made of composite materials. It is desirable to test specimens of these parts for compression strength.
There are machines available to accomplish this type of testing. For example, during compression testing the specimen, such as a flat, thin, homogeneous sample (as already demonstrated) or a composite, sometimes referred to as a "coupon", is supported by a conventional test specimen holder (also referred to as a fixture) between the grips of the testing machine. The machine compresses the coupon along its lengthwise direction until it fails, whereupon the testing machine provides a read-out of the force provided to fail the specimen. Conventional type testing devices provide external support for coupons along their lengthwise axes which is required during compression testing to prevent Euler column buckling. However, test coupons should be supported in a manner so that buckling is eliminated without interfering with the transmission of the full load to each specimen. Also, if the coupon is a composite it should be supported in a manner so that the sublaminate buckling of the specimen, or any natural failure mode of interest is not restricted. In addition, during compression testing it is desirable to attach an extensometer or strain gages to the coupon to measure the amount of deformation or strain during compression.
These conventional fixtures have a large contact area with the test specimen which inherently transfers some of the test load to the fixture itself, resulting in an erroneous overestimate of the coupon's compressive strength. Also, an opening must be cut into the fixture to low for open and filled hole compression tests. This makes it difficult to obtain valid strain measurements in the vicinity of the opening.
Existing compression fixtures support faces of the test specimen. While these fixtures restrict the Euler column buckling, they also restrict the valid sublaminate buckling failure mode of composite samples. Specimen designs based on stable column sections have been used to measure compression properties of composites. However, these specimens have very restrictive application and are not suitable for general use.
A number of conventional support devices has been disclosed in U.S. Patents, see REFERENCES CITED. For example, U.S. Pat. No. 683,184 by Rockwell shows a clamp having four rectangularly arranged blocks connected together in pairs by compression and expansion screws. In addition, U.S. Pat. No. 2,350,060 by Montgomery and U.S. Pat. No. 2,368,900 by Templin disclose compression testing devices for thin specimens that include a pair of T-shaped jaws having small diameter rollers to engage the surfaces of the specimen. And U.S. Pat. No. 4,840,070 by Rolfs et al., utilizes a laminate compression tester which includes a pair of adjustable stabilizing jaws having end segments that engage specimen grips of the testing machine. Finally U.S. Pat. No. 5,297,441 by Smith et al., describes three embodiments: The first embodiment supports the test piece along both of its lengthwise edges while engaging between the grips of the testing machine, in the second the apparatus supports the test piece along only one of its edges, and in the third embodiment, the apparatus includes grip plates which are mounted to the test machine and engage a portion of the test specimen while the remainder of the specimen is supported along its lengthwise edges by stabilizer plates.
All of the above described inventions, to some degree, restrain the applied force such that the force is neither fully allowed to be absorbed by the test piece nor is it accurately known. This results in an erroneous measurement of this force leading to an error in the determination of the compression strength of the test sample.